Integral field spectroscopy (IFS) is an observational method for obtaining spatially resolved spectra over a specific field of view (FoV) in a single exposure. In recent years, near-infrared IFS has gained importance in observing objects with strong dust attenuation or at a high redshift. One limitation of existing near-infrared IFS instruments is their relatively small FoV, less than 100 arcsec2, compared with optical instruments. Therefore, we developed a near-infrared (0.9 to 2.5 μm) image-slicer type integral field unit (IFU) with a larger FoV of 13.5×10.4 arcsec2 by matching a slice width to a typical seeing size of 0.4 arcsec. The IFU has a compact optical design utilizing off-axis ellipsoidal mirrors to reduce aberrations. Complex optical elements were fabricated using an ultra-precision cutting machine to achieve root mean square surface roughness of less than 10 nm and a P-V shape error of less than 300 nm. The ultra-precision machining can also simplify the alignment procedures. The on-sky performance evaluation confirmed that the image quality and the throughput of the IFU were as designed. In conclusion, we successfully developed a compact IFU utilizing an ultra-precision cutting technique, almost fulfilling the requirements.
This conference presentation was prepared for the Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation V conference at SPIE Astronomical Telescopes + Instrumentation, 2022.
SUNRISE III is an international balloon-borne solar observatory with a 1m diameter telescope and is scheduled to fly in 2022. The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is being developed as a focal plane instrument for SUNRISE III, which will perform multi-wavelength spectropolarimetric observations with high spatial and spectral resolution (0.21 arcsec and 2 × 105 ). The SCIP is a quasi-Littrow-type spectropolarimeter mainly composed of an echelle grating and two aspheric mirrors. A polarizing beam splitter is used for simultaneous measurement of p- and s-polarization. To ensure the imaging performance, correct the image rotation and shift, and avoid vignetting, we performed the optical alignment. The optical elements were firstly aligned by shimming with mechanical precision of 1-20 arcmin for tilt and 0.1-0.3 mm for shift using a coordinate measuring machine. After the mechanical alignment, we constructed the telecentric feed optical system to mimic the light distribution instrument of SUNRISE III telescope, which is called Image Stabilization and Light Distribution (ISLiD). To reduce the astigmatism, we measured the contrast of the spot in the spatial direction and the width of the spot in the wavelength direction and adjusted the two aspheric mirrors. For the correction of the image rotation, the Z-tilt of grating was adjusted with a wavelength tunable laser by evaluating the tilt of the slit image in the wavelength direction. The wavelength position in cameras was adjusted with tunable lasers and sunlight. We evaluated the modulation transfer function of SCIP using a Ronchi ruling target with a white light source.
We have developed an integral field unit (IFU) for the existing optical imaging spectrograph, Faint Object Camera And Spectrograph (FOCAS), on the Subaru telescope. FOCAS IFU finally saw a first light on March 2nd, 2018, and started the common use from 2019. In order to observe faint targets like distant galaxies, our IFU has a coarse sampling comparable to the best seeing size and high throughput. The field of view is 13.4 10.0 arcsec2 which is divided into 23 slices with the width of 0.435 arcsec. Our IFU has a slit separated by about 5.2 arcmin from an object field in order to simultaneously obtain a sky spectrum. We confirmed that the image quality is good enough for the 0.435-arcsec slice width and the best seeing size of 0.4 arcsec. Mean and median throughput of the IFU are respectively 85.0% and 87.3%. However some fields show lower throughput due to misalignment of the IFU optics and the worst throughput is 61.9% at one field corner. Flat fielding error is almost within ±3%, but worse errors are found at the low-throughput region. The worst error is 9% at the lowest throughput region.
We have been developed a lens-integrated superconducting camera for millimeter and submillimeter astronomy. High-purity silicon (Si) is suitable for the lens array of the microwave kinetic inductance detector camera due to its high refractive index and low dielectric loss at low temperatures. The camera is an antenna-coupled Al coplanar waveguide on a Si substrate. Thus the lens and the device are made of the same material. We report a fabrication method of a 721-pixel Si lens array with an antireflection (AR) coating. The Si lens array was fabricated with an ultraprecision cutting machine. It uses TiAlN-coated carbide end mills attached with a high-speed spindle. The shape accuracy was less than 50 μm peak-to-valley and the surface roughness was arithmetic average roughness (Ra) of 1.8 μm. The mixed epoxy was used as an AR coating to adjust the refractive index. It was shaved to yield a thickness of 185 μm for 220 GHz. Narrow grooves were made between the lenses to prevent cracking due to the different thermal expansion coefficients of Si and the epoxy. The surface roughness of the AR coating was Ra of 2.4 to 4.2 μm.
We present the concept, design, fabrication, and evaluation of a new deformable mirror (DM), which is latchable, compact, and designed to be applicable for cryogenic environments. The main body of a prototype DM was fabricated from a monolithic cuboid of aluminum using wire electrical discharge machining (EDM). A flexible structure was constructed inside the block by 3-dimensionally crossed hollowing using the EDM. The prototype has 6 × 6 channels, and its volume is 27 mm × 27 mm × 30 mm. The mirror was formed on the surface of the aluminum block using a highprecision NC lathe. The surface figure of the mirror was evaluated and 34 nm rms was obtained. The evaluated surface roughness for the center and off-center areas of the mirror was 9.2 nm rms and 7.6 nm rms, respectively Screws set at the back of the block deform the mirror via springs and the internal flexible structure. We present our first demonstration of deformation of the mirror carried out at ambient temperature. The relationship between the displacement of the screws and the deformation of the mirror was evaluated. Consequently, a linear relationship was confirmed, and no significant hysteresis was found. The application of such mirrors to telescopes used for various different objectives is discussed. We conclude that a DM based on our concept can be used for wavefront correction of space-borne telescopes, especially in the infrared wavelength region.
We have carried out the trial production of small format (n=5) image slicer aiming to obtain the technical verification of the Integral Field Unit (IFU) that can be equipped to the next generation infrared instruments such as TMT/MICHI and SPICA/SMI. Our goal is to achieve stable pseudo slit image with high efficiency. Here we report the results of the assembly of the image slicer unit and the non-cryogenic evaluation system of the pseudo slit image quality in the infrared.
We are developing an integral field unit (IFU) with an image slicer for the existing optical spectrograph, Faint Object Camera And Spectrograph (FOCAS), on the Subaru Telescope. The slice width is 0.43 arcsec, the slice number is 23, and the field of view is 13.5 × 9.89 arcsec2. Sky spectrum separated by about 5.7 arcmin from an object field can be simultaneously obtained, which allows us precise background subtraction. Slice mirrors, pupil mirrors and slit mirrors are all glass, and their mirror surfaces are fabricated by polishing. Our IFU is about 200 mm × 300 mm × 80 mm in size and 1 kg in weight. It is installed into a mask storage in FOCAS along with one or two mask plates, and inserted into the optical path by using the existing mask exchange mechanism. This concept allow us flexible operation such as Targets of Opportunity observations. High reflectivity of multilayer dielectric coatings offers high throughput (>80%) of the IFU. In this paper, we will report a final optical layout, its performances, and results of prototyping works.
Wide field cryogenic optics and millimeter-wave Microwave Kinetic Inductance Detector (MKID) cameras with Si lens array have been developed. MKID is a Cooper-pair breaking photon detector and consists of supercon- ducting resonators which enable microwave (~GHz) frequency multiplexing. Antenna-coupled Aluminum CPW resonators are put in a line on a Si substrate to be read by a pair of coaxial cables. A 220 GHz - 600 pixels MKID camera with anti-reflection (AR) coated Si lens has been demonstrated in an 0.1 K cryostat. A compact cryogenic system with high refractive index materials has been developed for the MKID camera.
We have been developing a lens-integrated superconducting camera for millimeter and submillimeter astronomy. High-purity silicon (Si) is suitable for the lens array of the Microwave Kinetic Inductance Detector (MKID) camera due to the high refractive index and the low dielectric loss at low temperature. The camera is antenna-coupled Al coplanar waveguides on a Si substrate. Thus the lens and the device are made of the same material. We report a fabrication method of 721 pixel Si lens array with anti-reflection coating. The Si lens array was fabricated with an ultra-precision cutting machine. It uses TiAlN coated carbide end mills attached with a high-speed spindle. The shape accuracy was less than 50 μm peak-to-valley and the surface roughness was Ra 1.8 μm. The mixed epoxy was used as anti-reflection coating to adjust the refractive index. It was shaved to make the thickness of 185 μm for 220 GHz. Narrow grooves were made between the lenses to prevent cracking due to different thermal expansion coefficients of Si and the epoxy. The surface roughness of the anti-reflection coating was Ra 2.4 ~ 4.2 μm.
We report the restraint deformation and the corrosion protection of gold deposited aluminum mirrors for mid-infrared
instruments. To evaluate the deformation of the aluminum mirrors by thermal shrinkage, monitoring measurement of the
surface of a mirror has been carried out in the cooling cycles from the room temperature to 100 K. The result showed
that the effect of the deformation was reduced to one fourth if the mirror was screwed with spring washers.
We have explored an effective way to prevent the mirror from being galvanically corroded. A number of samples have
been prepared by changing the coating conditions, such as inserting an insulation layer, making a multi-layer and overcoating
water blocking layer, or carrying out precision cleaning before coating. Precision cleaning before the deposition
and protecting coat with SiO over the gold layer seemed to be effective in blocking corrosion of the aluminum. The SiO
over-coated mirror has survived the cooling test for the mid-infrared use and approximately 1 percent decrease in the
reflectance has been detected at 6-25 microns compared to gold deposited mirror without coating.
An image slicer is highly in demand for an integral field unit (IFU) spectrograph of the next generation infrared
telescopes. This paper reports the results of the trial production of three key optical elements for a small format (number
of slice; n=5) image slicer, i.e. monolithic slice mirrors, monolithic pupil mirrors and monolithic pseudo slit mirrors. We
have demonstrated that sufficiently high processing accuracy and mirror surface accuracy for infrared observations are
achieved for each optical element based on our super precision cutting techniques.
Mid-infrared Medium Resolution Spectrometer (MRS) is one of the key spectroscopic modules of Mid-
Infrared Camera and Spectrometers (MCS) that will be onboard SPICA. MRS is an Echelle Grating
spectrometer designed to observe a number of fine structure lines of ions and atoms, molecular lines, and
band features stemming from solid particles and dust grains of the interstellar and circumstellar
medium in the mid-infrared wavelength range. MRS consists of two channels; the shorter wavelength
channel (MRS-S) covers the spectral range from 12.2 to 23.0 micron with a spectral resolution power of
R~1900-3000 and the longer wavelength channel (MRS-L) covers from 23.0 to 37.5 micron with
R~1100-1500 on the basis of the latest results of the optical design. The distinctive functions of the
MRS are (1) a dichroic beam splitter equipped in the fore-optics, by which the same field of view is
shared between the two channels, and (2) the small format image slicer as the integral field unit
installed in each channel. These functions enable us to collect continuous 12-38 micron spectra of both
the point-like and diffuse sources reliably with a single exposure pointed observation. In this paper, the
specifications and the expected performance of the MRS are summarized on the basis of the latest
results of the optical design. The latest progress in the development of the key technological elements,
such as the Dichroic Beam Splitter and the Small Format Monolithic Slice Mirrors, are also reported.
We are developing an integral field unit (IFU) with an image slicer for the existing optical imaging spectrograph,
Faint Object Camera And Spectrograph (FOCAS), on the Subaru Telescope. Basic optical design has already
finished. The slice width is 0.4 arcsec, slice number is 24, and field of view is 13.5x 9.6 arcsec. Sky spectra
separated by about 3 arcmin from an object field can be simultaneously obtained, which allows us precise
background subtraction. The IFU will be installed as a mask plate and set by the mask exchanger mechanism
of FOCAS. Slice mirrors, pupil mirrors and slit mirrors are all made of glass, and their mirror surfaces are
fabricated by polishing. Multilayer dielectric reflective coating with high reflectivity (< 98%) is made on each
mirror surface. Slicer IFU consists of many mirrors which need to be arraigned with high accuracy. For such
alignment, we will make alignment jigs and mirror holders made with high accuracy. Some pupil mirrors need
off-axis ellipsoidal surfaces to reduce aberration. We are conducting some prototyping works including slice
mirrors, an off-axis ellipsoidal surface, alignment jigs and a mirror support. In this paper, we will introduce our
project and show those prototyping works.
Mid Infrared Spectrometer with an Image Slicer (MIRSIS) is a compact mid-infrared spectrometer with an image slicer
as a testbed of techniques for efficient observations with next generation telescopes. MIRSIS is a 10-micron band
spectrometer for ground-based observations. Optics of MIRSIS is mostly composed of reflective ones. A key point of the
development of MIRSIS is a fabrication of slicer optics, which consists of slice mirrors, pupil mirrors and pseudo slit
mirrors. It is necessary to develop fabrication technique of slicer optics, because shapes and alignment of these mirrors
are special. Here it is also important to choose the design matched to the processing method. In this paper, we report our
fabrication of the slicer optics elements in detail. As a result, we achieved the slice mirror with the micro-roughness of
RMS 12nm and the angle accuracy of under 0.0041deg, the pupil mirror with the micro-roughness of RMS 20nm and
the shape accuracy of PV 3micron, and the pseudo slit mirror with the angle accuracy of 0.02deg. All of the parts
fabricated satisfy the required specification.
Mid-Infrared Spectrometer with an Image Slicer (MIRSIS) is a 10micron band spectrometer for ground-based
observations. Based on the optical design reported in Okamoto et al. (2006), we recently developed most of
optical elements and their mounts. There, we adopted designs based on an ultra-precision cut for the slice mirrors
and the pupil mirrors. We also designed and partly manufactured the optical parts with switching/adjusting
mechanism with cryogenic step motors. Since MIRSIS has a very complicated stereoscopic configuration of
optical elements, we developed a method to adjust the optical alignment where relative positional markers and
a three-dimensional measuring system are combined. We confirmed that we can achieve position and angular
adjustment with error down to 0.1mm and 0.05degree through alignment test with a pair of mirrors.
A spectrometer with integral field units on large optical/infrared telescopes enables efficient spectroscopy of moderately extended objects. In future mid-infrared observations with 30m class telescopes, where circumstellar disks larger than the spatial resolution will be major targets, such efficient observations are strongly desirable. Here we present an optical design of our new N-band image slicing spectrometer to test basic techniques for future image slicing spectrometers on larger telescopes. Our prototype image slicer follows the idea of the advanced image slicer considering not only object images but also pupil images and is optimized for the N-band (10 micron atmospheric window). Five slicing mirrors and five pupil mirrors are used to slice the field of view and make a rearranged pseudo slit image. The pseudo slit image is collimated, dispersed by a grating, and imaged on a Si:As 320x240 array. For the slicing mirrors, we plan to use polished stainless mirrors of 300 micron width. The spectral resolution is set as about 200. We plan to put an imaging optics module for target aquisition in addition to the simple image slicer module. The whole optics is designed to be compact (about 600mm x 450mm x 300 mm), which will allow us to make test observations easily with various telescopes.
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